Sample-and-hold circuit

ABSTRACT

A sample-and-hold circuit which comprises a first input amplifier, a switch connecting a second amplifier to the output of the first amplifier and a pair of diodes which prevents saturation of the first amplifier during the hold period by closing a negative feedback path around the first amplifier when the signal level exceeds the diode conduction voltage.

United States Patent 1191 Hill Aug. 14, 1973 [54] SAMPLE-AND-HOLD CIRCUIT 3,586,880 6/1971 Fitzwater, Jr. 328/151 X 1 3,588,531 6/1971 Bjor 307/235 X [751 lnvemorn'llThwsand Oaks 3,564,287 2/1971 Todd 307/235 Cahf- 3,602,825 8/1971 Senior 307/235 R x 3,686,577 8/1972 Fruhauf 307/235 R X [73 l Assignee: The United States of America as OTHER PUBLICATIONS re resented b the Secretar of the E y y Kennedy, Variable Threshold c6nne1," IBM Tech. Discl. Bull. V01; 8, No. 4, p. 692-693, 9/1965. [22] plied 1972 Bjorkman, Peak Picking ,& Noise Suppression Cir? 21] App] 231 310 cuitry," IBM Tech. Discl. Bull.;, Vol. 9, No. 6, p.

R, Hansen Burst Mode Amplifier Tech .307/251 Disc]. BulL, V01. 14, N6. 7, p. 2196-2197, 12/1971. ["36 CL... Heam for Amps 53 Field of Search 307/235 R, 235 A, Electronic Products Magazine, p. 54-55, 6/21/1971.

307/246, 251, 229, 230; 328/150, 151 Bosswetter, Sample -Hold Circuit is Inexpensive &

Stable," Electronic Design, p. 94, 7/4/1968. [56] References Cited UNITED STATES PATENTS Primary Examiner-John W. Huckert 3,430,072 2/1969 Stevens 328/151 x Assistant N 511821108 2,924,709 2/ 1960 Morrill 328/147 A t rney-Richard S. Sc1asc1a et :11. 3,474,259 10/1969 Rodgers 307/235 X 3,555,298 l/1971 Neelands 307/235 [57] ABSTRACT 3,413,491 11/1968 Reeves 307/235 A 1 3,304,507 2/1967 Weckes et a1. 328/151 A 'l 'l comlg'ses mput 3,363,1 13 1/1968 Bedingfield 307/238 a mmlecmg a to F 3,328,705 6/1967 Eubanks 307/235 A x outPu ampllfie' and 3 P of P whlch 3,259,760 7/ 1966 Morey et a]. 307/235 A prevents saturanon of the first amplifier during the hold 3,479,534 11/1969 Miller 307/235 x period y closing a negative feedback P around the 3,482,174 12/1969 James 307/240 X first amplifier when the signal level exceeds the diode 3,543,169 11/1970 Hill 328/151 conduction voltage. 3,553,492 1/1971 Bugay 307/235 5 Claims 3 rawi g Figures R 2 R 3 90.09 9.09 l /r: /0

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C 2 1 N 9 2 8pF 2 v +5 v 2.5 566 0 I 12v I 2 V H 5 V H BACKGROUND OF THE INVENTION This invention relates to sample-anddiold circuitsand more particularly to a system for taking an accurate instantaneous sample of an input; signal and hold ing this value as a fixed output voltage until the next sample is taken. An important object is'to maintain isolation between the signal source and the output load.

Sample-and-hold circuits have a wide range of application in the field of electronics. Examples are analogto'digital converters, zero order interpolation in sampled data systems, gated. A GC circuits, etc. PAM and PC M telemetry systems. are sampled, data systems, as is the pulse orthogonal multiplexing (POM). type telemetry system which in some respects is superior to the other telemetry systems. sample-andrhold circuits are used in all sampled data systems. The sample-and-hold circuit of this invention is uniquely suited to the multiplexer portion of the. POM type telemetry system.

Many different approaches and. configurations. have been used to achieve the sample-and-hold function. These prior approaches all sufferfrom one or more dis-. advantages. They donot present a high constant resistive load to the signal. source at all times, or they donot guarantee adequate protection of thesignal source inthe event of circuitfailure. Also they may." require more than one switch and; may require two, closely aligned coincident. sample pulses rather than. just one sample pulse. These prior approaches, also, may not provide a low output impedance, especially as an integral part of the sample-and-hold circuit, and. the output may expo nentially approach. final: value duringsample time, as contrasted. with the faster response of a second) order system.

SUMMARY OF THE INVENTION The sample-and-hold circuit of this invention. coma put of the first amplifier, a feedback path between the two amplifiers, and a pair of oppositely poled diodes.

connected between the feedback path and the output of the first amplifier. The; switch: is closed during the sample periodand the. capacitor is charged; in, the hold period the switch is opened and. the sampled, voltage is held on the capacitor until'needed. The diode pairprevents saturation ofthe. first amplifier during the hold period by closing; a negative feedback patharound the first amplifier when. the signal level exceeds the diode conduction voltage.

The present circuit was designed" to. sample sensitive operational circuits and transducers ina remotetest ehicle. Two of the prime requirments off such, sampleand-hold circuits are: thatrit: presents a highandcons tant resistive load to the signal source atall times, and that in. the event of? failure of the sample-and-hold: circuit an appreciable resistive isolation, be. provided be-v tween the signal source and the active components comprising the sample-and-hold, circuit.

This invention embodies essentially all the desirable features for a sample-and-holdj system. in. one. relatively. simple circuit. In the present circuit, a high; constant re-. sistance load with a ground potential: reference: is, pres; ented to the signal. source, at, all times, and the signal. source is protected; by preventing abnormal loading thereof in the-eventofcircuit failure. A low output impedance is presented to the load at all times, both during hold and sample periods. The circuit has the high speed of response that is associated with a second order system, permits optimum adjustment of the damping factor, and1 has the high precision associated with operational amplifiers. Only one SPST switch is required in the present system and this system can be implemented with integrated. circuits. Further,ythis circuit prevents saturation of the input amplifier during the hold period,

and has means to adjust for optimum transient response to minimize time to final value. The circuit provides precision samplingof the input voltage with the sample time short relative to the sample period, and negligible decay in. voltage during maximum hold time.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction. with the accompanying drawings wherein.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a simple schematic drawing of the circuit of the present invention.

FIG. 2. is a schematic diagram of an input operational amplifier which can be used for the sample-andrhold circuit of FIG. 1.

FIG. 3. is a detailed schematic diagram of a preferred embodiment. of the sample-and-hold circuit of FIG. 1

DESCRIPTION OF THE PREFERRED EMBODIMENTS The schematic diagram of FIG. I shows two operationalamplifiersj however, only the input amplifier Al. requires the. precision characteristics normally associated with an operational amplifier. A schematic diagram of a suitable amplifier for input amplifier Al is shown in FIG. 2. The output amplifier A2 operates as a. non-inverting amplifier with a gainof (R4 R5)/R4; The important requirements of output amplifier A2 are a high input impedance and a low output impedance; high load current capability is not required. The requirements of output amplifier A2 are satisfied by means of a simple field effect transistor (FET) compound circuit as shown in the output section of the detailed schematic diagram of FIG. 3. The values associated: with the circuit components in each of the figures are given; merely by way of example.

A sample of the input signal is takenby closing the sampling switch SW1 for a time which is short relative to. the rate of change of the input data. At the end of where a, represents the value of the input voltage a,,(t) at the endof the sampling interval. Because the resis tors are selectedfor unity gain, the output voltage e, at the end of the sampling time is -a,.

Examination of the. circuit shown in FIG. I. shows that the foregoing statements will be true only if nocurrent flows in. diodesv D1. and D2 during sampling. This current will be negligible provided that the voltage acrossthe diodes is less than the junction barrierpoten' tial. For silicon diodes, this potential is several tenths of a volt, except, at extremely high temperatures. The condition for zero voltage across the diodes is given by the, following equation:

During the sampling interval, capacitor C1 is charged to the appropriate voltage to bring the output voltage to a,,. When the switch SW1 opens, this charge on capacitor C1 will be maintained because of the high input impedance of the output amplifier; hence the output voltage e will remain at -a,,.

Diodes D1 and D2 serve two functions. First, they prevent the input operational amplifier A1 from being saturated during the hold period. Otherwise this would occur, since amplifier Al would then be operating with open-loop gain, and between sample pulses small changes in the input voltage a,,(t) would overdrive A1. Diodes D1 and D2 prevent this because they close the feedback path through resistor R2 as soon as the output from amplifier Al changes sufficiently to cause diode conduction. (Saturation of Al would be undesirable since the recovery time would reduce the speed of response.) With switch SW1 open, resistor R3 is no longer in the feedback path and represents only a small additional load on each amplifier.

The second function of diodes D1 and D2 is that of maintaining the input summing junction at virtual ground. Thus the input data source is presented with a constant resistive load to ground. Series resistor R1 provides the added advantage of isolating the data source, or input source, from the active components of the sample-and-hold circuit. This prevents possible shorting or serious loading of the data source if failure occurs in the sample-and-hold circuit. The input load resistance is, therefore, at all times referenced to a fixed voltage, i.e., virtual ground. The use of diodes D1 and D2 in conjunction with resistors R2, R3, R4 and R5 to prevent saturation of amplifier Al during the hold period is an important feature of this circuit.

Proper operation of the sample-and-hold circuit shown in FIG. 1 depends upon proper operation of switch SW1. A schematic of switch SW1 and an account of its operation is hereinafter given in FIG. 3 and the discussion thereof.

The diagram of FIG. 2 illustrates an FET operational amplifier of the type which is used for input amplifier Al forlthe sample-and-hold circuit shown in FIGS. 1 and 3.

FIG. 3 is a detailed schematic diagram of the multiplexer sample-and-hold circuit of FIG. 1 also showing components of the sampling switch SW1. The functions and required characteristics of the components of switch SW1 which include an N-channel FET (UC- 200) plus associated components; diodes D3 and D4, resistor R6, and capacitor C2 are also described in copenidng US. Pat. application, Ser. No. 231,314, filed Mar. 2, 1972, for High Frequency Field Effect Transistor Switch. During the negative state of sampling pulse H, which is obtained from a one-shot multivibrator for example, sampling switch SW1 is open, and during the positive state of sampling pulse H, switch SW1 is closed. When the sampling pulse H is in its negative state, current flows through diode D3 and resistor R6. For fast closing of sampling switch SW1, the minority storage charge in diode D3 must be sufficient to discharge the gate-to-channel capacitance of the FET of SW] and the capacitance of diode D4. When switch SW1 is opened, this gate-to-channel capacitance is recharged. The necessary charge is taken from holding capacitor C1, thus altering its final voltage. The func-' switch SW1 is closed. This reduces'the tansients associated with opening of the switch by preventing minority carrier stored charge accumulation. This in turn reduces the required magnitude of capacitor C2, thus making this a less critical adjustment. The required characteristics of diode D4 are low junction capacitance, low stored charge, and high conductance. The hot carrier diode HP 2800 serves well here for diode ln order for the transients at output e to decay to a sufficiently low value at the end of the sample period, a suitable damping factor is necessary. The trim capacitor C3 permits adjustments of the damping factor.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

l. A sample-and-hold system for taking an accurate, instantaneous sample of an input signal and hold this value as a fixed output voltage until the next sample is taken, comprising:

a. an input operational amplifier connected to an input signal source;

b. an output operational amplifier;

c. a sample switch means connecting the output of said input amplifier to the input of said output amplifier;

d. capacitor means in parallel with said sample switch means also connected to the input of said output amplifier;

e. a feedback path between the output of said output amplifier and the input to said input amplifier;

f. a pair of oppositely poled diodes connected between said feedback path and the output of said input amplifier;

g. said switch means being closed during the sample period of operation of the system charging said capacitor means with the sampled voltage, said switch means being opened during the hold period and the sampled voltage being held on said capacitor means until needed, providing precision sampling of the input voltage with the sample time short relative to the sample period, and negligible decay in voltage during maximum hold time;

h. said diode pair preventing saturation of said input amplifier during the hold period by closing a negative feedback path around the input amplifier when the signal level exceeds diode conduction voltage;

. resistive means provided for isolating the input signal source from active components of the sampleand-hold system, the system presenting a high and constant resistive load to the protecting signal source at all times, prtecting the signal source by preventing abnormal loading thereof in the event of circuit failure; and

4. A system as in claim 1 wherein said output operational amplifier is a simple field effect transistor compound circuit.

5. A system as in claim 1 wherein to diode pair maintains the input 0t said input operational amplifier at virtual ground for presenting the input signal source with a constant resistive load to ground. 

1. A sample-and-hold system for taking an accurate, instantaneous sample of an input signal and hold this value as a fixed output voltage until the next sample is taken, comprising: a. an input operational amplifier connected to an input signal source; b. an output operational amplifier; c. a sample switch means connecting the output of said input amplifier to the input of said output amplifier; d. capacitor means in parallel with said sample switch means also connected to the input of said output amplifier; e. a feedback path between the output of said output amplifier and the input to said input amplifier; f. a pair of oppositely poled diodes connected between said feedback path and the output of said input amplifier; g. said switch means being closed during the sample period of operation of the system charging said capacitor means with the sampled voltage, said switch means being opened during the hold period and the sampled voltage being held on said capacitor means until needed, providing precision sampling of the input voltage with the sample time short relative to the sample period, and negligible decay in voltage during maximum hold time; h. said diode pair preventing saturation of said input amplifier during the hold period by closing a negative feedback path around the input amplifier when the signal level exceeds diode conduction voltage; i. resistive means provided for isolating the input signal source from active components of the sample-and-hold system, the system presenting a high and constant resistive load to the protecting signal source at all times, prtecting the signal source by preventing abnormal loading thereof in the event of circuit failure; and J. said system presents a low output impedance to the load at all times, during both sample and hold periods.
 2. A system as in claim 1 wherein said sampling switch means is a field effect transistor switch circuit.
 3. A system as in claim 1 wherein said output operational amplifier has a high input impedance and a low output impedance.
 4. A system as in claim 1 wherein said output operational amplifier is a Simple field effect transistor compound circuit.
 5. A system as in claim 1 wherein to diode pair maintains the input ot said input operational amplifier at virtual ground for presenting the input signal source with a constant resistive load to ground. 